1. Field of the Invention
This invention relates to systems and components that must create varied synthesized clock frequencies with abilities for high-frequency modulation rates with low phase noise jitter. More particularly this invention relates to systems and components that which include clock recovery systems that track very high frequency components of data jitter when extracting clocking signals from data streams.
2. Description of the Related Art
Multi-gigabit per second communications links have recently been standardized in numerous applications; each of which have slightly different requirements and therefore slightly different specifications. Applications from 150 Mbit/sec to 12 Gbit/sec are commonplace and each requires development and evaluation in their individual environment. A single clock recovery module that offers the dynamic range and flexibility to allow convenient settings to adapt the module for use in each individual environment is desired. Key to this achievement, is the creation of a phase-locked loop within a clock recovery module that operates at all the required data rates and that allows adjustable phase locked loop transfer characteristics (bandwidth and peaking/damping) to allow emulation of a “golden” phase locked loop in that application. In order to make such a phase locked loop that covers today's modern range of data rates and loop transfer characteristics with sufficient quality, it is vital that a new voltage-controlled oscillator be invented that offers pure oscillations (low jitter) as well as high-bandwidth frequency controllability.
Present voltage-control oscillators used in phase-locked loops employ various ways to cause oscillations and various ways to modulate the frequency of these oscillations. Examples of these devices include semiconductor ring oscillators typically tuned by electrically adjusting the time delay in the path around the ring, Inductor-Capacitor (LC) tank circuits who's capacitance can be varied electronically causing different oscillation frequencies, and Yttrium Iron Garnet (YIG) oscillators who's oscillations are caused by amercing the YIG sphere in a magnetic field building a microwave tank circuit allowing a modulation of the magnetic field to cause a modulation in the oscillator's frequency.
Of these and many other oscillator technologies, the YIG-style oscillator offers a large dynamic range of oscillation frequencies and a very pure oscillation tone (high-Q)—owing to its mechanical structure. However, as is common in any oscillator where the frequency of oscillation is adjusted by changing an input control voltage, any attempt to support high-frequency modulation of the oscillator's frequency (through the input control voltage) results in high phase noise of the oscillator's output. This is because an oscillator that can oscillate at a wide-range of frequencies will, by definition, respond to a large range of input control frequency spectra and an unavoidable large amount of energy will also accompany any real control voltage signal as noise. This noise will also fall into the allowed input control frequency spectra and any voltages found on the input control frequency (intended or not) will change the output frequency of the oscillator. It is well understood that the output phase noise of the entire phased-locked loop is the integral of the frequency variations. Thus, in-band noise energy will be integrated and will cause the classic VCO to have large phase noise when wide adjusting bandwidths are desired. Phase noise translates directly to jitter which is one of the key performance metrics for a clock output derived from the phased-lock loop inside a clock recovery unit.
Fundamentally, then, all phase-locked loops that directly employ a singular voltage controlled oscillator (VCO) where output phase of the oscillator is locked and adjusted by changing the frequency of oscillation of the VCO are plagued with added phase noise when attempting high-frequency modulation caused by the realization that the output phase is the integral of the controlled frequency. As is understood by those skilled in the art, the high frequency modulation element cannot be “inside” the oscillation loop (where integral of frequency relates to output phase noise).
In order to overcome the above, what is needed is the high frequency modulation element to be outside the loop (e.g. in the form of a phase modulator) where the phase error is not the integral of the control signal but rather just directly related to it. In accordance with the spirit and scope of this invention such a structure is provided.